Maou was once a great demon lord who was defeated by a hero. After ten years of slumber, Maou awakens to claim their revenge, but finds their stature now diminutive, and their old enemy nothing but a one-room slacker. Maou decides to move in with Max in hopes of helping him return to the old nemesis he used to be in hopes of claiming vengeance. He was resurrected with an undeveloped body due to a lack of accumulated magical power, but when he goes out, he transforms into a beautiful girl with a nice body.

(Source: Level 1 Demon Lord and One Room Hero Wiki)